1. Field of the Invention
The present invention relates to a thread supplying device of a sewing machine. Particularly, the present invention relates to a thread supplying device applied to, for example, the case of supplying a needle thread, being called an upper thread, to a needle producing a combined movement, namely up and down, and from side to side, in a lock stitch (zigzag stitch) sewing machine, and the case of supplying a needle thread, and upper and lower looper threads to a needle moving vertically, and upper and lower loopers, respectively, in an over lock machine. This sewing machine is disposed on a thread supply path extending from a thread supplying part such as a thread cone to a movable sewing member including loopers, so that it can sequentially feed and supply the threads as a sewing operation advances.
2. Description of Related Art
As a thread supplying device of a sewing machine of this type, a generally known one has the following construction. That is, in order to avoid an excess or insufficient feed of a thread even when the type of a thread or the speed of sewing is changed, a pressure regulating mechanism enabling adjustments of the pinch pressure of the thread in a tension disk, a pair of rollers, a tension pulley, and the like, as well as adjustments of the feed resistance of the thread, is disposed at any point along a thread supply path. An operator adjusts or sets in advance the pinch pressure (hereinafter referred to as pressure regulation) of the thread by means of the pressure regulating mechanism before starting a sewing operation in accordance with the type of the thread and the speed of supply of the thread.
In the conventional thread supplying device having the above-mentioned pressure adjusting mechanism, a laborious pressure adjusting operation is required whenever the type of a thread used is changed as a sewing operation is advanced. Particularly, when using a flexible thread, the amount of feed of the thread varies greatly as the sewing machine is speeded up and down. It is therefore difficult to perform an adequate handling, and hence the amount of feed of the thread is unstable. In addition, if the rotational speed of the sewing machine is changed rapidly and the amount of feed of a fabric is increased or decreased greatly, it is difficult to have the amount of feed of the thread sufficiently track its great increase or decrease, from the viewpoint of the response characteristic of the above-mentioned pressure adjusting mechanism. It is therefore unavoidable that the amount of feed of the thread becomes excessive or insufficient, failing to stabilize stitch performance.
On the other hand, for example, Japanese Unexamined Patent Publication No. 2000-202179 discloses one having a rotary thread feeding mechanism. That is, a pulley is provided in a motor rotatable in synchronism with and working with the vertical movement of a needle, and the advance and retraction of upper and lower loopers. By driving the motor in forward or reverse direction, an upper thread and a looper thread wound around the pulley are fed or pulled back (returned).
In textile machinery such as a knitting machine, a hosiery machine, and a bobbin winding machine, for example, Japanese Unexamined Patent Publication No. 11-322195 discloses the following construction. That is, thread tension detecting apparatus and thread speed measuring apparatus are provided together, and the speed of supply of the thread supplied to the textile machinery is automatically controlled by controlling the number of revolutions of a drive pulley so that both of tension and speed of the thread are maintained constantly.
However, the conventional rotary thread supplying device as disclosed in the above Publication No. 2000-202179 lacks tracking because the feed and pullback of the thread are alternately repeated by the forward or reverse rotation of the thread feeding mechanism, namely being so-called intermittent thread feeding. Especially, if the feed of a fabric is largely increased or decreased with a rapid change in the rotational speed in a high-speed rotation range, the feed of the thread cannot be stabilized. Further, due to the alternative repetition of the feed and pullback of the thread, the thread extending between the thread supply part including the thread cone and the movable sewing member including the needle and the loopers is liable to swing widely, and the swing thereof may occur frequently, causing the likelihood of the thread mating together and being caught. This may increase the likelihood that the stitch performance itself will become unstable.
On the other hand, the thread supplying device in the textile machinery such as the knitting machine as disclosed in the above Patent Publication No. 11-322195, the thread is supplied to the machine (the knitting machine) by controlling the number of revolutions of the drive pulley based on the detections of the thread tension and the thread speed. When this thread supplying device is applied to a sewing machine, it fails to track the high-speed rotation of the sewing machine, making it impossible to avoid unstable stitch performance. This is because the thread supplying device of the sewing machine requires a different function from the knitting machine or the like in which a thread supplied causes less change in thread tension, that is, a complicated control of rotation along with the intermittent driving. Specifically, in response to thread tensions to be detected in the tension state and the tension released state of the thread because of the vertical movement of the needle along with the rotation of the sewing machine, and the advance and retraction of the loopers, the drive pulley is rotated at high speed, or stopped or rotated at low speed, in order that the thread is fed when applying tension, and the feed of the thread is stopped or delayed when releasing the tension.